Known exhaust gas purification systems to be mounted on engine powered machines include one provided with an NOx removal catalyst arranged in an exhaust pipe and a reducing agent feeder for injecting a reducing agent, which is stored in a reducing agent tank, from a side upstream of the arranged location of the NOx removal catalyst to selectively subject nitrogen oxides in exhaust gases to reduction treatment with the reducing agent in the presence of the NOx removal catalyst such that the nitrogen oxides are decomposed into harmless nitrogen gas and water. Usable as the reducing agent is a urea water that undergoes hydrolysis in the exhaust pipe and is changed into ammonia having good reactivity with nitrogen oxides, an aqueous ammonia solution, gas oil containing hydrocarbons as principal components, or the like.
A reducing agent such as a urea water or aqueous ammonia solution freezes at low temperatures and produces ammonia gas having an offensive odor at high temperatures, and therefore, requires appropriate temperature control while being stored in a reducing agent tank. Conventionally-proposed means for maintaining a reducing agent, which is stored in a reducing agent tank, within an appropriate temperature range include the one which is provided with a reducing agent heater for introducing an engine cooling medium into the reducing agent tank to heat with heat of the engine cooling medium the reducing agent stored in the reducing agent tank and an on/off device for opening/closing a cooling medium flow passage that guides the engine cooling medium to the reducing agent heater, and which, when the reducing agent is in a frozen state, switches the on/off device into an open state to thaw the frozen reducing agent with heat of the engine cooling medium and, when the reducing agent has been heated to a predetermined temperature above a thaw temperature of the reducing agent, switches the on/off device into a closed state to cut off an inflow of thermal energy such that overheating of the reducing agent is prevented (see, for example, Patent Document 1).    Patent Document 1: JP-A-2005-90431